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FL-2100 Input tuning circuit

Discussion in 'Amateur Radio Amplifiers' started by NN3Y, Jul 1, 2019.

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  1. NN3Y

    NN3Y XML Subscriber QRZ Page


    This is my first thread in this forum. I Apologize in advance if there has been similar post an I have missed it. Recently got back into the hobby and started serious DX'ing. Acquired this linear some time ago and mostly used it on 20m and had no complains. As I tried it on 40m, realized my IC756 III showing an excessive SWR and manual tune of IC756 could not tune to the input of the linear. That being said, I realized that the input tuning cirquit is not resonant on the mid-range of the 40m band but much lower frequency. FL2100 manual describes alignment on the input circuits but when took it apart, realized there were no tuning slugs on the input coils. Has anybody experienced that the input circuit caps may be changing value with age? Any suggestion is welcomed.

  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    There are no slugs in any of the input tuning coils?

    In the FL-2100Bs I've worked on there were tuning slugs in all of them (5).

    Are you sure?
  3. NN3Y

    NN3Y XML Subscriber QRZ Page

    I'm Positive. Well, I purchased it used and had to do some work on it but this section looked "virgin". I have to look again but almost positive now that the coils were hollow.
  4. G3YRO

    G3YRO Ham Member QRZ Page

    The ceramic former Input Coils ARE hollow . . . but each one has an adjustable ferrite core to resonate it.

    The ferrite is connected to a metal screw thread, that pokes through the chassis to the underside, so that you can easily adjust them simply by removing the bottom panel.

    You can see these adjustment screws along the edge of the chassis, in the top left of this picture:


    And by the way, the only difference between the FL2100 and the FL2100B is the colour of the paint ! The FL2100 matches the FT101 Mk1 and Mk2 . . . the FL2100B matches the FT101B, E and F.

    Roger G3YRO
    Last edited: Jul 2, 2019
    W1TRY likes this.
  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    That photo is how I remember all the FL-2100's I've ever seen or worked on. Yes, adjustable only by removing the bottom cover, but if they didn't have ferrite cores in them, there would be nothing to adjust!
  6. G3YRO

    G3YRO Ham Member QRZ Page

    Well I find it odd that somebody might remove them ! (maybe the OP just didn't see them?)

    Roger G3YRO
  7. NN3Y

    NN3Y XML Subscriber QRZ Page

    Thank you all for the feed back. I have been looking on the top of the coils. I'll take another look tonight. All responses are very much appreciated. I'll keep you in the loop.

  8. NN3Y

    NN3Y XML Subscriber QRZ Page

    Thank you all, finally found them and was able to bring the center frequency on the 7Mhz input coil closer and my IC 756 was able to tune just fine. Noticed that there is a very little change whit in the entire trip of the slug. just enough to bring it closer.

  9. G3YRO

    G3YRO Ham Member QRZ Page

    Well those coils have to handle 100 watts of RF . . . so they are basically almost resonant on their own - the ferrite in each one is quite small (so it doesn't saturate) . . . so they only give a slight change in inductance for fine tuning. (unlike - say - a receiver front-end coil)

    Anyway - glad you located the adjustments OK.

    Yaesu put the adjustment slugs underneath, so that all you have to do is remove the bottom cover, rather than take the whole case off.

    Incidentally, I always found it annoying that Yaesu DIDN'T use an FT101 style case on the FL2100, with a lift up lid, so you could easily change PA Valves etc. I actually had the square version of this Amp - the FL2000B - which DID have a lift-up lid. (but electrically identical)

    Roger G3YRO
    NN3Y likes this.
  10. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Yaesu did this better than Heathkit; but an even better idea would be to locate the tunable networks at the rear panel with a tuning tool access hole for each one so no covers have to be removed at all -- a la all the Ameritron amps.

    What Henry did in some models was even nicer: The networks are "inside" and you do have to remove the top cover to get at them, but you don't have to remove the inner shield cover over the tubes so with the first (cosmetic) top cover removed, you're not exposed to B+. Then, the input tuning networks were in plug-in shielded assemblies that plugged in like vacuum tubes, with the adjustable slugs on top for easy access. But the brilliance was making them all plug-in networks, so if you wished to operate the amp on frequencies outside the tuning range of the normal five input networks, you could buy or build a new one for a different range and plug it in.

    That was a really good idea. Henry provided instructions for modifying them for "outside the ham bands" operation, to optimize for MARS or other military/commercial frequencies. This added some cost, of course, but they sure sold a lot of amplifiers.
    NN3Y likes this.

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